This invention relates to erosion control devices and methods adapted to check shoreline erosion to allow beach material to accrete.
In the United States and other countries, miles of beaches are annually subjected to severe erosion which literally washes away beachfront and exposes higher ground and valuable property to wave action. If left unchecked, wave and current action erodes the property and undermines foundations of shoreline buildings and houses causing them to topple into the water.
Erosion of this type has been exacerbated and often created by man-made structures. In one typical situation, a pier or jetty is constructed at a river mouth and extends perpendicular from the shoreline into the water to form a navigation channel into the mouth of the river. Littoral or near shore currents impinge upon the sides of the pier deflecting the currents away from shore. These currents typically carry sand which would otherwise be deposited near shore between naturally occurring sandbars extending parallel to the shore and the beach. However, since the currents are deflected away from shore, the sand is carried out to deep water, robbing the beach area of sand which would otherwise deposit there.
Furthermore, the deflected currents actually wash away protective sandbars. Sandbars are critical to beach protection since they dissipate waves and littoral currents. When sandbars erode, the beachfront in the area of the eroded sandbar is exposed to much stronger currents and waves, causing even more severe beach erosion.
Beachfront property owners often spend tens of thousands of dollars each to construct seawalls or revetments on and parallel to the beach in an attempt to stop such erosion. Such attempts, however, serve only to accelerate erosion. Seawalls and revetments only direct the energy of the waves and currents downwardly to the foundation of the seawall or revetment, which scours sand and rock at the foot of the seawall or revetment structure and which ultimately causes the structure to fall into the water. Such downward scouring also deepens the water in the area and allows sediments to be carried away from the littoral zone, leading to even more severe erosion.
Another approach typically taken to attempt to stop such erosion is to position piles, groins or other such structures perpendicular to shore. Such structures are invariably constructed so that they extend into the water from the beach and upward several feet above the surface of the water. Again, littoral currents running parallel or at acute angles to the beach deflect from these structures and carry sand seaward. Also, the waves associated with them are reflected downwardly in the immediate vicinity of each of these structures, eddying and scouring sand and rock on the foot or base of each structure. This eddying eventually undermines the structure and causes it to topple into the water. There have been attempts to reduce the effects of scouring at the bases of the structures by building structures directly in bedrock. However, such construction is extremely expensive as it requires underwater excavation. Such construction is also almost financially prohibitive, especially for the average property owner, in most of the Great Lakes region for bedrock is covered by as much as several hundred feet of unconsolidated clay, sand and gravel.
In addition to the above problems, the increasing wave height and current velocity in a littoral zone created by these "solutions" leads to other types of erosion and foundation problems. It has recently been observed that the weight of a large wave can force water below it into granular, sandy material along the ocean or lake bottom. As water is forced into the granular material, it provides a lubricating water film between the grains and liquifies sandy material below the waves such that currents, if they have sufficient velocity, will wash the liquified material away, or erosion control devices placed on the material will gradually sink into the liquified material. When the devices sink, of course, they lose whatever effectiveness they may have had.
Finally, all of the described devices ruin the aesthetics and desired recreational characteristics of the beach. Because they cause water to deepen and wave energy to increase, these devices create unsightly, scarp-like erosion formations on the beach above the waterline. The deeper water and the upwardly projecting structures also pose hazards for swimmers.